Born near Abisola, 21 July, 1414; died 12 Aug., 1484. His parents were poor, and while still a child he was destined for the Franciscan order. Later he studied philosophy and theology with great success at the University of Pavia, and lectured at Padua, Bologna, Pavia, Siena, and Florence, having amongst other eminent disciples the famous Cardinal Bessarion. After filling the post of procurator of his order in Rome and Provincial of Liguria, he was in 1467 created Cardinal of S. Pietro in Vincoli by Paul II. Whatever leisure he now had was devoted to theology, and in 1470 he published a treatise on the Precious blood and a work on the Immaculate Conception, in which latter he endeavoured to prove that Aquinas and Scotus, though differing in words, were really of one mind upon the question. The conclave which assembled on the death of Paul II elected him pope, and he ascended the chair of St. Peter as Sixtus IV.

His first thought was the prosecution of the war against the Turks, and legates were appointed for France, Spain, Germany, Hungary, and Poland, with the hope of enkindling enthusiasm in these countries. The crusade, however, achieved little beyond the bringing back to Rome of twenty-five Turkish prisoners, who were paraded in triumph through the streets of the city. Sixtus continued the policy of his predecessor Paul II with regard to France, and denounced Louis XI for insisting on the royal consent being given before papal decrees could be published in his kingdom. He also made an effort like his predecessor for the reunion of the Russian Church with Rome, but his negotiations were without result. He now turned his attention almost exclusively to Italian politics, and fell more and more under his dominating passion of nepotism, heaping riches and favours on his unworthy relations. In 1478 took place the famous conspiracy of the Pazzi, planned by the pope's nephew - Cardinal Rafael Riario - to overthrow the Medici and bring Florence under the Riarii. The pope was cognizant of the plot, though probably not of the intention to assassinate, and even had Florence under interdict because it rose in fury against the conspirators and brutal murderers of Giuliano de' Medici. He now entered upon a two years' war with Florence, and encouraged the Venetians to attack Ferrara, which he wished to obtain for his nephew Girolamo Riario. Ercole d'Este, attacked by Venice, found allies in almost every Italian state, and Ludovico Sforza, upon whom the pope relied for support, did nothing to help him. The allied princes forced Sixtus to make peace, and the chagrin which this caused him is said to have hastened his death.

Henceforth, until the Reformation, the secular interests of the papacy were of paramount importance. The attitude of Sixtus towards the conspiracy of the Pazzi, his wars and treachery, his promotion to the highest offices in the Church of such men as Pietro and Girolamo are blots upon his career. Nevertheless, there is a praiseworthy side to his pontificate. He took measures to suppress abuses in the Inquisition, vigorously opposed the Waldenses, and annulled the decrees of the Council of Constance. He was a patron of arts and letters, building the famous Sistine Chapel, the Sistine Bridge across the Tiber, and becoming the second founder of the Vatican Library. Under him Rome once more became habitable, and he did much to improve the sanitary conditions of the city. He brought down water from the Quirinal to the Fountain of Trevi, and began a transformation of the city which death alone hindered him from completing. In his private life Sixtus IV was blameless. The gross accusations brought against him by his enemy Infessura have no foundation; his worst vice was nepotism, and his greatest misfortune was that he was destined to be placed at the head of the States of the Church at a time when Italy was emerging from the era of the republics, and territorial princes like the pope were forced to do battle with the great despots.